1. Field of the Invention
This invention relates to improved wetting solution for contact lenses. In a specific aspect the novel wetting solutions of this invention have improved disinfecting and/or preserving properties.
2. Description of the Prior Art
Contact lenses in wide use fall into two categories: the hard or rigid corneal type lenses formed from materials prepared by polymerization of acrylic esters, such as polymethyl methacrylate (PMMA), and gel, hydrogel or soft type lenses made of polymerized hydrophilic or hydrophobic monomers, such as 2-hydroxyethyl methacrylate (HEMA).
Cleaning, storing and wetting solutions are required for both the hard and the soft types of contact lenses. These solutions usually contain a wetting aeent to enhance wearer's comfort in combination with a germicide or preservative, a viscosity builder and salts that adjust the tonicity of the solution to make them compatible with the osmolality of the tear fluid. U.S. Pat. No. 4,323,467 describes wetting solutions that may contain gelatin as a viscosity builder and the gelatin is used in an amount sufficient to achieve viscosity of the solution of 15-750CPS at 25.degree. C.
The hard acrylic type of contact lens is highly durable and, since the lenses do not absorb appreciable amounts of water, the selection of suitable disinfecting agents, cleaning agents or other lens care compounds is relatively non-critical.
However, unlike hard lenses, the soft type of contact lens and certain of the newer gas permeable hard contact lenses have a tendency to bind and concentrate significantly more fluids, environmental pollutants, water impurities, as well as antimicrobial agents found in lens care solutions. In most instances, the low levels of the ingredients in lens care solutions do not lead to eye tissue irritation when used properly. Nevertheless, because of the inherent binding action of protein deposits and soft lens materials, disinfecting agents and preservatives tend to build up on lens surfaces and become concentrated to potentially hazardous levels, such that corneal inflammation and other eye tissue irritation can result.
Previous efforts to alleviate the problem of binding and concentrating disinfectants and preservatives onto contact lens surfaces and reducing the potential for eye tissue irritation have not been totally satisfactory. For example, in spite of low toxicity levels, not all disinfectants are compatible for use with all types of contact lenses. Many hard lens disinfecting and preservative solutions contain benzalkonium chloride or chlorobutanol. Although they are effective antibacterial agents, their use can result in a loss of lens hydrophilic properties, cause solution instability or may even lack compatibility with certain types of hard lenses, e.g., high silicon content.
Other antibacterial agents were found to be more compatible with contact lenses and exhibit less binding on lens surfaces. Such agents are disclosed in U.S. Pat. Nos. 4,354,952, 4,361,548 and British Pat. No. 1,432,345. However, these compositions have exhibited serious disadvantages, and there is a need for improved disinfecting and preservative solutions which are compatible for use with most types of contact lenses while maintaining both a high level of antibacterial activity and low order of toxicity to eye tissue with little or no binding or concentrating of the disinfecting agent onto lens surfaces.
We have found that gelatin can be used in contact lens care solutions in amounts such that the viscosity of the solution is substantially lower than the viscosity of prior art contact lens care solutions and that the gelatin in our solutions is a highly effective wetting agent or demulcent. We have also found that the gelatin-containing solutions of our invention can also contain microbicidal effective amounts of certain biguanides or water-soluble salts thereof in such relatively small amounts that many of the problems of the prior art preservative and disinfectants are overcome.